1. Field of the Invention
The present invention relates to an apparatus and method for controlling a power source used in portable electronic equipment. More particularly, the present invention relates to an apparatus and method for controlling charge/discharge cycles of a battery power source used in portable electronic equipment, such as a portable type computer.
2. Discussion of the Background
In recent years, various types of mobile electronic equipment, such as a notebook type computer or a personal digital assistant device (PDA) have been developed. These electronic devices are usually designed to accommodate both power from a battery and an AC adaptor. The battery is needed in consideration of outdoor use when a commercial AC power source is unavailable.
Usually, a secondary battery, such as a nickel hydrogen battery and a lithium ion battery is used in this type of electronic equipment. When an AC adaptor is coupled to the electronic device, the power supply source is automatically switched from the battery to the AC adaptor. The battery is also charged through the AC adaptor.
Most of the battery-powered electronic devices include battery controlling functions, such as a low-battery detecting function and a battery state control function. The low-battery detecting function is used to detect a battery state when a battery voltage drops lower than a prescribed threshold voltage. The low-battery detecting function is also used to warn a user as to the low battery state. The low-battery detecting function may be achieved by using a discharge characteristics curve of the secondary battery.
FIG. 10 shows a typical discharge characteristics curve of the secondary battery. The characteristic curve illustrates the output voltage of the secondary battery decreasing as a discharge time of the battery increases. Usually, when the voltage of the secondary battery drops below a prescribed level, the performance of the battery deteriorates and diminishes.
To prevent such a defect state, a discharge end voltage (V.sub.END) is usually defined as an allowed discharge limit voltage of the secondary battery. When the voltage of the secondary battery approaches the discharge end voltage (V.sub.END), it is necessary to take certain precautions. The precautions include, for example, switching to the AC adaptor power source or saving the data. This is necessary to prevent a loss of data due to a lack of battery power. For this purpose, the low-battery detecting function should be able to warn a user about the dangerously low state of the battery. In addition, the precautions should be performed while there is still a sufficient amount of time left for the user to switch to the AC adaptor power source or to save the data before the secondary battery reaches the discharge end voltage (V.sub.END).
Accordingly, a low-battery detection voltage (V.sub.LB) is set for detecting the low battery state at a voltage value that will occur at a prescribed time prior to reaching the discharge end voltage (V.sub.END). As depicted in FIG. 10, by using the discharge characteristics of the secondary battery, the voltage value of the low-battery detection voltage (V.sub.LB) is set at a prescribed time before the discharge end voltage (V.sub.END).
The battery state control function controls the default state of the battery by using battery information recorded in a recording means provided in the battery unit. The recorded battery information contains information, such as a realizable capacity and a residual capacity.
The realizable capacity is a parameter used for indicating a battery performance. It indicates a maximum value of the discharge capacity of the battery from a fully charged state to the discharge end voltage (V.sub.END). The residual capacity is used for calculating a usable residual time of the device.
Each time the battery completes a charging/discharging cycle, the battery state control function updates the battery information. The information is updated to a latest realizable capacity value or a latest residual capacity value. The residual capacity is determined by calculating a difference between the discharging capacity and the realizable capacity stored in the recording means. Similarly, the realizable capacity is determined by calculating a difference between the charging capacity and the residual capacity. The values are also updated before the start of next charging or discharging cycle of the battery.
FIG. 11 illustrates repeated battery charge/discharge cycles for a fully charged new battery (i.e., the battery has no deterioration) installed in an electronic device. The new battery discharges when it is used as the power source. Further, the battery is charged when an AC adaptor is used as the power source. During the operation of the electronic equipment, the battery repeats these charge/discharge cycles. With reference to FIG. 11, these cycles will now be explained.
Battery discharge cycle 1:
After installing the fully charged new battery into the electronic device, a power source switch is turned on. At this initial operation time, the realizable capacity value and the residual capacity value of the battery are read from the memory. In this case, at first, the realizable capacity value of the battery equals the residual capacity value of the battery. The residual capacity value read from the memory is used for calculating the usable residual time of the electronic device. A user of the device is informed of the calculated usable residual time by using an appropriate indication, such as a display.
The battery is discharged when it is used during the operation of the electronic device. The discharge of the battery is stopped when the battery powered operation is turned off or the device is switched to the AC adaptor power source. In these cases, the used discharge capacity value that has discharged from the battery up to the switching time is subtracted from the initially stored realizable capacity value (i.e., the realizable capacity minus the used discharge capacity). The result of the subtraction is stored as a new residual capacity value in the memory of the battery unit.
Battery charge/discharge cycle 2 (This is also the same as the battery charge/discharge cycle 3):
By switching to the AC adaptor power source, the battery begins to be charged. When it has been fully charged, the charging capacity value at that time is added to the residual capacity value previously stored in the memory prior to the battery being charged. In addition, the result of the addition is written into the memory device of the battery as the latest realizable capacity value of the battery. When the battery is again used as the power source, the battery begins to discharge. When the discharge of the battery reaches a low-battery state, the low-battery detecting function transmits a warning to the user. As the power source is switched from the battery to the AC adaptor, the battery stops discharging. At this stage, the used discharge capacity value that has discharged from the battery up to that time is subtracted from the stored realizable capacity value. The result of the subtraction is written into the memory device of the battery as the latest residual capacity value.
As explained above, in the conventional method, the battery power supply is immediately stopped once when the power source is turned off or switched to the AC adaptor power source. This is performed without any consideration of whether or not the low-battery state has been detected. In addition, the latest residual capacity value is calculated by the subtraction of the realizable capacity value minus the discharge capacity value and written in the memory. Also, when the charging of the battery has been completed, the charged capacity value is added to the stored residual capacity value for calculating the latest realizable capacity value of the battery.
However, the performance of the secondary battery used with a portable personal computer naturally diminishes with the number of repetitions of these charge/discharge cycles. That is, the secondary battery has a characteristic that the actual realizable capacity value gradually declines depending upon the number of repetitions of the charge/discharge cycles.
Consequently, the discharged capacity value from the fully charged state of the battery to a certain residual capacity value may not always be equal to the charging capacity value. The charging capacity value is determined from when the battery is charged from the low battery state to the fully charged state. That is, after repeating the charge/discharge cycles many times, the actual realizable capacity value of the battery becomes smaller than the calculated realizable capacity value.
Therefore, the conventional calculation method for obtaining the latest values of the realizable capacity and the residual capacity includes an error. Although this error is relatively small when the repetition of charge/discharge cycles is small, as the repetition of charge/discharge cycles increases, the errors or differences severely increase. Then it becomes impossible to correctly calculate the residual time of the battery.
The discharge characteristics of the battery also vary as the repetition of the charge/discharge cycles increases. Consequently, it is necessary to dynamically update the aforementioned low-battery detection voltage (V.sub.LB) to match such a change in the discharge characteristics.
However, in the conventional method, the low-battery detection voltage (V.sub.LB) is always maintained at a constant value. Therefore, the low-battery state may not be detected even when the battery has reached the low-battery state.
As mentioned above, in the conventional method, the battery state is controlled without considering the characteristics of the battery in which the realizable capacity gradually declines with an increase in repetitions of charge/discharge cycles of the battery. Accordingly, as the repetition of the charge/discharge cycles increases, the errors between the calculated values and actual values of the realizable capacity and the residual capacity become inevitably larger. As a result, it becomes impossible to know the correct residual time for using the battery as the power source of the electronic device. This is a severe disadvantage with the conventional apparatus and method for controlling the power source.
Further, because the conventional method uses a constant value for the low-battery detecting voltage and does not consider changes in the discharge characteristics of the battery, it becomes impossible to detect the low-battery state correctly. This is another severe disadvantage with the conventional method and apparatus for controlling the power source.